Founder Jeremy Toeman offered a personal promise on Thursday. "So there you have it, as long as Nudgemail is available to the public, it's going to be free," Toeman said in a statement to users obtained by CNET.

Nudgemail has built a loyal user base in the four and a half years since it launched. The service is essentially a reminder engine built into email, allowing users to send any email they've received to Nudgemail and have it automatically sent back to them as a reminder at a designated time. Users can also send an email with a day, date or time to Nudgemail. So, for instance, if a task needs to be completed by Wednesday, users would send the details to "wednesday@nudgemail.com." Another task that needs to be done on, say, October 11, could be sent to "october11@nudgemail.com." Once those days roll around, an email is received from Nudgemail listing the item and task details input the day it was sent.

Nudgemail requires no plug-ins and works in every email client on any device, since users simply send an email to Nudgemail and the service takes it from there. If a user doesn't feel like handling a task, Nudgemail includes a snooze button to deal with it at a later time. Although it doesn't provide exact figures, the company said hundreds of thousands of people have used Nudgemail.

Nudgemail is one of many reminder-based services, though it attempts to achieve its goal of keeping people on-task in a different way. For example, Wunderlist, which Microsoft recently acquired, also provides to-dos to users, though it does so in its own standalone service. Nudgemail's pitch is that it's built directly into email.

Nudgemail's move was unexpected. Previously, the service, which had been in a "free public beta," said on its site that once it came out of beta, a premium version would be available to complement the free offering.

"The paid version is how we'll stay in business and keep offering this great service," the site read for years.

The course change prompts the question of how Nudgemail will actually stay in business. According to Toeman, he's going to follow a "pay what you want" model. Nudgemail will not force users to pay anything to use its service, but those who want to can donate whatever they'd like to help defray costs.

"Fundamentally, we believe the concept of email-based reminders should be fundamental to email," Toeman told CNET. "The very concept that the primary method of digital communications is broken for so many people just plain irks us. Further, we don't think people should have to pay to make their inboxes work." He added, "We've had so many of our users ask if they can financially help support the effort, we're taking them up on it."

Nudgemail will be adding a page to help users make payments to the company. Those options will include a one-time payment, a monthly payment or an annual option. Nudgemail will use digital-payment platform Stripe to process payments.